摘要/Abstract
冰-水界面动力学性质在冰形核、生长、表界面熔化中扮演核心角色, 长期以来一直被广泛关注. 然而, 受限水体系中冰-水界面的动力学性质却鲜有报道. 本工作利用平衡态分子动力学模拟方法和受限固-液两相平衡模拟技术, 对两种水模型(恒定偶极矩、可极化)描述的单分子层受限冰-水两相平衡体系中的一维固-液界线开展研究. 通过对一维受限冰-水界线的追踪, 计算了其热涨落波动的振幅与时间自关联函数色散谱, 进而计算一系列固-液界线动力学性质. 冰-水界线波动在短波长区域复合了快、慢两种不同时间尺度的弛豫过程, 在长波长区域则由慢弛豫过程主导. 相比块体冰-水界面体系, 以Rayleigh波为主的高频微观物理过程更多地参与了一维冰-水界线的动力学弛豫. 我们发现冰-水界线波动弛豫特征衰减时间的波矢依赖关系符合现有固-液界面动力学理论, 但一维界线弛豫的特征衰减时间比二维界面体系低了一个数量级左右. 计算了两种水模型体系冰-水界线的动力学系数, 并与块体冰-水界面比较, 发现受限冰-水(固-液)界线动力学系数远高于块体冰-水界面体系. 我们推测水分子转动自由度在受限腔中被强烈压制可能是导致受限体系超快冰-水(固-液)相变速率的主要原因. 本工作将在受限水体系超快相变(储能、传感)器件的设计工作中提供一定的理论指导意义.
关键词: 受限水, 固-液界面, 毛细波动力学, 结晶动力学, 分子动力学
The ice-water interface's kinetics has long been paid much attention because of its central role in ice nucleation, growth and surface/interface melting. However, there exist few studies focused on the kinetics of the confined ice-water interface. In this paper, we employ the equilibrium molecular dynamics simulation and the phase equilibrium technique for the confined water and ice to study the 1D crystal-melt coexistence line of an equilibrium mono-layer ice-water coexistence system, described by two well-known water molecule models, i.e., constant dipole moment TIP4P/2005 model and polarizable SWM4-NDP water model. The mono-layer ice-water phase equilibria are confined in the hydrophobic slab pore of 0.65 nm, under 0.5 GPa lateral pressure. By tracking the 1D ice-water coexistence line's position, we calculate the power spectrum of the equilibrium line fluctuation and the time-dependent autocorrelation function of the line fluctuation Fourier amplitudes and then calculate a series of kinetic properties of the 1D crystal-melt coexistence line. We demonstrate that the processes involved in the relaxation of the crystal-melt coexistence line fluctuation with long wavelengths are coupled with a fast and a slow decay process characterized by two distinct time scales, while just the slow decay processes dominate the crystal-melt coexistence line fluctuation with short wavelengths. By comparing with bulk ice-water interface systems, we find that the high-frequency processes such as Rayleigh waves participate more in the relaxation of the 1D crystal-melt coexistence line fluctuation. We see that the wave vector dependence of the characteristic relaxation time (of the crystal-melt line fluctuation) is consistent with the crystal-melt interface's existing kinetic theory. Nevertheless, the characteristic relaxation time of the 1D crystal-melt coexistence line relaxation is around one order of magnitude lower than that of the 2D bulk ice-water interface system. We calculate the kinetic coefficients of the 1D crystal-melt coexistence line for the two water model systems, compare with the bulk interface systems, and find the kinetic coefficient of the confined ice-water (crystal-melt) coexistence line is much higher than that of the bulk ice-water interface system. The significant increase in the magnitude of the kinetic coefficient of the confined 1D ice-water coexistence line system may be understood by the substantial suppression of the rotational degree of freedom of the confined water molecules. The current achievement of the fundamental insight and simulation results could potentially facilitate the theoretical advancements in designing new devices of ultrafast phase-change (energy storage, sensor) devices based on confined water systems.
Key words: confined water, solid-liquid interface, capillary wave dynamics, crystallization kinetics, molecular dynamics
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